IL-1 beta-converting enzyme (ICE) is present and functional in human alveolar macrophages: macrophage IL-1 beta release limitation is ICE independent

Tissue macrophages readily produce intracellular pro-IL-1beta in response to stimuli such as LPS, but are limited in mature IL-1beta release compared with blood monocytes. The mechanism of this IL-1beta control may provide important insights into the physiology of IL-1beta at the tissue level. Since it has been hypothesized that IL-1beta processing by the IL-1beta-converting enzyme (ICE) regulates IL-1beta release, we compared human alveolar macrophages and human blood monocytes for relative ICE expression and activation. Using immunoblots and enzyme-linked immunoassay for ICE, we demonstrate that alveolar macrophages do not differ from blood monocytes in antigenic p45 ICE. Furthermore, an indirect assay for functional ICE documents similar ICE activities in both monocytes and alveolar macrophages, i.e., similar concentrations of soluble synthetic ICE inhibitor (IC50 values of 0.3 +/- 0.01 and 0.6 +/- 0.2 microM, respectively) are required to block mature IL-1beta generation. However, as has been reported for THP-1 myelomonocytic cells, neither alveolar macrophages nor blood monocytes contain directly quantifiable levels of functional ICE forms (p22/p20 and p10) when assayed by immunoblots or by a sensitive capture ELISA that uses an irreversible, biotinylated ICE inhibitor. These findings document that the macrophage limitation in mature IL-1beta release is not due to a lack of ICE or to an inability to activate ICE. Finally, using a staged release assay, the time to half-maximum mature IL-1beta release is significantly depressed in macrophages compared with that in monocytes. Taken together, these findings suggest that macrophage IL-1beta export is regulated independently of ICE activation.